Headspacing on PTR-91 - I need a sanity check

Bob The Great · August 7th, 2009, 07:57 PM

I'm attempting to check the headspace on my PTR-91, so my first step was to purchas .308 GO and NOGO gages. To my surprise, I found that my rifle will lock up on both GO and NOGO just fine.

After an initial "ah, crap!" moment, I did some more research and found this interesting page.

HK Weapon Systems Forums: checking headspace

The second post in that thread says that headspace on these rifles really isn't important, and that "bolt gap" between the rear of the bolt and the bolt carrier face is the important variable to measure.

Quote:

There is actually a thread on this in the General discussions forum called "How to check PTR bolt gap?"
The HK roller locking design is a bit different than other rifle designs. The chambers on most of the ones I've seen have been short by about .008" or so, which is I think normal for these due to the way they lock up. When measured on an empty chamber, the bolt gap on these rifles should be between .004" and .018". This gap is the result of the bolt head locking against the breach end of the barrel, which causes the rollers on either side of the bolt head to lock against the trunion. The rollers engage a tapered locking piece which is attached to the bolt carrier. Depending on how far the barrel protrudes into the trunion, the rollers will act on the tapered locking piece to keep the bolt gap within the desired range. So bolt gap and headspace are really two entirely different things. The two are related though, on a loaded chamber, the bolt gap will increase because the short chamber design cases the bolt head to lock up against the base of the cartridge rather than the breach end of the barrel. The taper on the locking piece is a 4 to 1, meaning that every movement inward of the rollers will result in 4 times that amount in rearward movement of the bolt carrier. So a chamber that is short by .008" will result in an increase in bolt gap by .032". Heres a pic that shows a cutaway of the HK action:

Unfortunately the little bits and pieces aren't labled, but you can see all of the important ones. Surrounding the cartridge is of course the barrel, surrounding that is the trunion. At the base of the cartridge is the bolt head, one of the rollers can be seen locked into the recessed area in the trunion. On the opposite side of the rooler is the locking piece, barely visible. Just behind that, about a third of the length back from the front edge of the mag well is where bolt gap is measured, between the bolt head and bolt carrier.

So to make a long story short, don't worry about headspace. As long as your bolt gap is between .004" and .018" on an empty chamber you should be good to go. Just to be safe, I'd measure it every hundred rounds or so for the first 1000 rounds. If you don't see more than a few thousandths loss of gap during that time you should be OK. Some of the PTR's seem to have come with faulty trunions that distort over time and allow the bolt gap to diminish very rapidly. There is a thread on this in the general topics forum as well: PTR91 Bolt Gap?

Hope this helps,

Mike

My bolt gap measures thusly:

Empty Chamber - .011 (In spec per the post quoted above)
GO Gage - .018
NOGO Gage - .029

In my thinking, the fact that the bolt gap increases when the GO gage (minimum sized cartridge) is chambered means that the gage is bearing up against the chamber shoulder and guarantees that the headspace is not too great.

If the bolt gap had remained the same, it would mean that the GO gage had not hit the chamber shoulder, which would indicate possible excessive headspace.

Am I thinking about this correctly? By this line of logic, my rifle has in-spec headspacing and is perfectly safe.

Cuda66 · August 7th, 2009, 09:27 PM

Quote:

Originally Posted by Bob The Great

I'm attempting to check the headspace on my PTR-91, so my first step was to purchas .308 GO and NOGO gages. To my surprise, I found that my rifle will lock up on both GO and NOGO just fine.

After an initial "ah, crap!" moment, I did some more research and found this interesting page.

HK Weapon Systems Forums: checking headspace

The second post in that thread says that headspace on these rifles really isn't important, and that "bolt gap" between the rear of the bolt and the bolt carrier face is the important variable to measure.

My bolt gap measures thusly:

Empty Chamber - .011 (In spec per the post quoted above)
GO Gage - .018
NOGO Gage - .029

In my thinking, the fact that the bolt gap increases when the GO gage (minimum sized cartridge) is chambered means that the gage is bearing up against the chamber shoulder and guarantees that the headspace is not too great.

If the bolt gap had remained the same, it would mean that the GO gage had not hit the chamber shoulder, which would indicate possible excessive headspace.

Am I thinking about this correctly? By this line of logic, my rifle has in-spec headspacing and is perfectly safe.

Yep.

Bob The Great · August 8th, 2009, 08:25 PM

Alright, I guess that'll do. Thanks.

August 8th, 2009, 08:25 PM	#3
Bob The Great Distinguished Member Join Date: Sep 2006 Location: Slidell, LA Posts: 1,569	Alright, I guess that'll do. Thanks. __________________ "A well-educated electorate, being necessary to the continuance of a free state, the right of the people to keep and read books shall not be infringed." Is this hard to understand? Then why does it get unintelligible to some people when 5 little words are changed?

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